As apparatuses for oscillating laser to be used for cutting of materials or various measurements, for example, a gas laser apparatus for oscillating a gas laser is widely used. In the gas laser apparatus, a rare gas such as helium-neon (He—Ne) gas or argon (Ar) gas, and, for example, carbon dioxide gas (CO2 gas) or the like are enclosed in a conduit of a gas laser tube, which is a circular-shaped cylinder, and plasma is generated in the conduit. Then, by exciting, for example, carbon dioxide gas molecules (CO2 molecules) by energy of the plasma, light of a specific wavelength is emitted from the carbon dioxide gas molecules (CO2 molecules). For materials used for the gas laser tube, a high airtightness and also a high transparency to a high-frequency electric power for generating plasma are required, and thus for example, materials such as quartz glass are widely used.
Plasma consists of charged particles such as electron or ion, or excited molecules or atoms, which react with each other while intricately moving, and thus is in a state where a physical energy thereof is high and a chemical reactivity is also high. Accordingly, an inner surface of the conduit of the gas laser tube exposed to plasma is likely to be subjected to a physical damage due to energy of the plasma exposed and also a change due to a chemical reaction (hereinafter, the physical damage and the chemical change are collectively referred to as corrosion). Corrosion of the laser tube due to the plasma causes a service life of the gas laser tube to be decreased and thus causes durability of the gas laser apparatus to be deteriorated.
For example, in Patent Literature 1, a configuration for inhibiting corrosion of a laser tube due to plasma is proposed. Specifically, in Patent Literature 1, there is proposed a structure in which a protective film made of metal such as tungsten or molybdenum, or boron carbide is formed on an inner circumferential surface of a laser tube body made of, for example, aluminum nitride. In Patent Literature 1, such a protective film is formed by ion plating, CVD or otherwise.